MANCHESTER, England — The investigation into a suicide blast that killed at least 22 people at a pop concert dramatically widened Wednesday, with security services on two continents rounding up suspects amid fears that the bombmaker who devised the bolt-spewing source of the carnage remains at large.

The arrests stretched from the normally quiet lanes of a northern English town to the bustling streets of Tripoli, where Libyan officials said they had disrupted a planned attack by the suspected bomber’s brother.

British authorities identified Salman Abedi, shown in an undated photo from an unnamed source, as the suicide bomber who was responsible for Monday's explosion at a concert in Manchester, England. Associated Press

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But by day’s end, British authorities acknowledged that they remained vulnerable to a follow-up attack, with the nation’s state of alert stuck at “critical” – the highest possible level.

The sight of soldiers deploying at London landmarks such as Buckingham Palace and 10 Downing Street underscored the gravity of a threat that was known in general terms before Monday night’s explosion but has come sharply into focus in the 48 hours since.

The morning after the attack, police had said they believed that the suspect, 22-year-old Salman Abedi, a British citizen, had carried it out alone and had died in the blast he triggered.

INVESTIGATING ‘A NETWORK’

But in their statements Wednesday, authorities expressed growing confidence that Abedi – who had recently returned from a trip to Libya and may have also traveled to Syria – had been only one part of a web of plotters behind Britain’s worst terrorist attack in more than a decade.

“It’s very clear that this is a network we are investigating,” Greater Manchester Chief Constable Ian Hopkins said.

Hopkins said police were moving quickly to disrupt the group, carrying out raids across the city and arresting four people, including Abedi’s older brother, Ismail. A fifth suspect was later apprehended carrying “a suspicious package” in the town of Wigan, about 20 miles west of Manchester.

On Wednesday evening, authorities arrested a female suspect in Manchester and a man in the English Midlands town of Nuneaton, bringing to seven the number of people detained in Britain in connection with the blast. A raid by balaclava-wearing police at an apartment in central Manchester spawned speculation that authorities may have uncovered the location where the bomb was built, although that appeared to have been unfounded.

Monday’s explosion claimed victims as young as 8 and targeted fans of U.S. pop star Ariana Grande, who was performing at Manchester Arena.

In conflict-scarred Libya, counterterrorism authorities said they had arrested at least two additional members of Salman Abedi’s family, including a younger brother suspected of preparing an attack in Tripoli.

Ahmed Dagdoug, a spokesman for Libya’s counterterrorism Reda Force, said Hashem Abedi was arrested late Tuesday and is suspected of “planning to stage an attack in Tripoli.”

Dagdoug said Hashem Abedi had confessed to helping his brother prepare the Manchester attack. “Hashem has the same ideology as his brother,” Dagdoug said.

Abedi’s father, Ramadan, was arrested Wednesday, although it was not clear on what grounds. Ramadan Abedi had earlier asserted that his sons were innocent, telling the Associated Press that “we don’t believe in killing innocents. This is not us.”

He said Salman sounded “normal” when they last spoke five days ago. The elder Abedi said his son had planned to visit Saudi Arabia and then spend the Islamic holy month of Ramadan with family in Libya.

Dagdoug said Hashem Abedi had been in frequent contact with Salman Abedi and was aware of the plans to attack the concert. Dagdoug described Hashem Abedi as an operative of the Islamic State group, which has asserted responsibility for Monday’s blast.

SUSPECT ON RADAR ‘TO A POINT’

It was unclear whether investigators believed that Salman Abedi’s relatives were a key part of the network planning the Manchester attack. But authorities were increasingly exploring the emerging connections between Britain and Libya.

Salman Abedi, whose parents had emigrated from Libya to escape the rule of Moammar Gaddafi, was on the radar of British security services before Monday’s attack.

But Home Secretary Amber Rudd, the nation’s top domestic security official, suggested that he was not a major focus of any inquiries, telling the BBC that authorities had been aware of him only “to a point.”

Rudd said that Abedi had recently returned from Libya and that that was a focus of the investigators’ inquiry.

In a highly unusual public rebuke, she also slapped down U.S. authorities for leaking information about the investigation, calling it “irritating.”

But even as she did, key details about the investigation were emerging from other allied capitals.

Rudd’s French counterpart, Interior Minister Gerard Collomb, told broadcaster BFMTV that Abedi may have also gone to Syria and had “proven” links with the Islamic State.

Abedi was reported on Wednesday to have been a college dropout who had recently become radicalized. Security experts said it was unlikely that he coordinated the attack, and the BBC reported that he may have been “a mule” tasked with carrying out the bombing but had little role in creating the explosive or choosing the target.

Of particular concern to British investigators was the possibility that the bombmaker was still at large and may be planning to strike again.

Prime Minister Theresa May had cited the possibility of a broader network of plotters on Tuesday night when she raised Britain’s alert level from “severe” to “critical” and announced the deployment of troops to guard key sites.

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